Ultra-passivation of chromium containing alloy and methods of producing same

ABSTRACT

The invention concerns article having a surface oxide layer up to 20 nm thick, the surface oxide layer comprising chromium and cobalt oxides where the atomic ratio of Cr/Co is more than 3. The invention also concerns methods for treating a chromium containing material, said method comprising contacting said material with a gas plasma under conditions effective to oxidize at least a portion of the material; and contacting said material with an acid. The treated surface is corrosion resistant and can be used in orthopedic implants, especially the wear surface of the orthopedic implant to reduce wear, and other corrosive environment.

RELATED APPLICATION

This application claims benefit of U.S. Provisional Application No.60/956,778, filed Aug. 20, 2007, which is incorporated herein in itsentirety.

BACKGROUND OF THE INVENTION

Metal-on-metal (MOM) prosthesis implants have been around since the1960s. Presently, the use of metal-on-metal implants has been increasingdue to the fact that historically MOM systems have shown better than twoorders of magnitude less wear than metal-on-polyethylene (MOP) systems.However, in the case of MOM devices, there is still concern regardingthe long term effects of in vivo metal ion release due to corrosionand/or wear of the metal components.

The corrosion resistance of CoCrMo alloys is thought to be due to theexistence of a natural metal oxide layer on the alloy surface. The majorcomponents of this oxide layer are cobalt oxide and chromium oxide.Because chromium oxide is much more resistant to leaching by body fluidsthan cobalt oxide, the corrosion resistance of the alloy depends on thecontinuity, thickness and chromium oxide content of the layer.

In the orthopedic industry, CoCr implants are routinely treated byimmersion in 30% nitric acid solution at an elevated temperature (54°C.). It is believed by many in the industry that passivating CoCrmaterials with nitric or citric acid leads to the production of acorrosion resistant surface by forming a thin transparent Cr-oxide film.It is this layer that they believe imparts the corrosion resistance toCoCr implant materials. These beliefs are based on work published forStainless Steel passivation. In reality, the surface of a nitric(citric) acid passivated surface has a surface composition ofapproximately 50:50 Co and Cr.

MOM implants are also subject to weight loss caused by the continuousfriction of two contacted surfaces moving in body fluid. As thesesurfaces contact each other, the interaction of asperities or surfaceroughness causes wear.

To date, most efforts to reduce wear in MOM systems have focusedprimarily on reducing surface roughness and/or surface asperities. Inparticular, many investigators believe that removing surface “carbideasperities” will dramatically reduce break-in wear and yield a lowerwear system. Many attempts have been made to remove surface carbides viaenhanced polishing methods and heat treatment processes designed to“dissolve” the carbides into the alloy. These efforts, however, haveproduced little impact in terms of a significant reduction in break-inwear. Thus, there remains a need in the art to reduce break-in wear inprosthesis devices.

SUMMARY OF THE INVENTION

One aspect of the present invention concerns chromium containing, suchas CoCrMo, articles having a surface oxide layer up to 20 nm thick, saidsurface oxide layer comprising chromium and cobalt oxides where theatomic ratio of Cr/Co is >3. In some aspects, the invention concerns amethod comprising: contacting the chromium containing material with agas plasma under conditions effective to oxidize at least a portion ofthe material; and contacting the material with an acid under conditionseffective to form a robust Cr-oxide surface layer substantially thickerthan that produced using the conventional process described above.

The material can be contacted with the gas plasma before it is contactedwith the acid or the material can be contacted with the acid before andafter it is contacted with the gas plasma.

In some embodiments, the acid includes nitric acid or hydrochloric acid.

In one embodiment, the acid includes hydrochloric acid and the firstacid passivation treatment is followed by a second acid passivationtreatment using an acid comprising nitric acid.

The gas plasma used in the methods of the invention can be derived froman oxidative gas or from a mixture of gases comprising oxidative gas andoptionally one or more inert gases. One preferred oxidative gas isoxygen. Inert gases include nitrogen, argon and helium. In certainembodiments, the material is contacted with the gas plasma for about 5to about 120 minutes. In some embodiments, the contacting of thematerial with the gas plasma is performed at a power of about 100 toabout 1000 watts. In some embodiments, the mixture of gasses comprisesat least 3% (v/v) percent oxidative gas. Other embodiments contain 100%(v/v) percent oxidative gas.

The invention also concerns articles made by the methods describedherein. In some embodiments, the invention concerns articles comprisingcobalt and chromium, the article having a surface comprising oxideswhere the oxides are characterized with a Cr/Co>3 (as measured by XPS).In certain embodiments, the oxides are characterized by a Cr/Co˜7. Insome articles, the surface oxide is 3-8 nm thick. This highly enrichedchromium oxide layer impart a significantly higher corrosion resistanceversus the relatively thin (˜1-2 nm) passive film formed via theconventional passivation process.

Some articles are designed to be implantable into a mammal. Certain ofthese articles are joint replacement prosthesis or component thereof.Such prosthesis include hip or knee replacement prosthesis.

In some embodiments, the treated material is used on a wear surface,especially an orthopedic implant wear surface to reduce wear.

Some articles are very corrosion resistant and they can be used incorrosive environment, including acid or alkaline environments. Theinventive passivation film can reduce the ion release in simulated bodyfluid.

The invention also concerns methods comprising implanting a prosthesiscomprising chromium and cobalt in a human or animal body in contact withbone, wherein at least a portion of the prosthesis in contact with saidbone comprising a surface having an exterior oxide layer of up to 20 nmin thickness that is highly enriched in chromium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical wear graph for MOM prosthesis devices. The graphis divided into two phased: the initial run-in phase and the secondsteady state phase. The X-axis is the cycle number and the Y-axis is theaccumulative volume loss of the prosthesis. The slope of the graph isthe volume loss rate. The graph shows that a majority of the volume losshappens in the initial stage—run-in phase.

FIG. 2 shows XPS sputter depth profile data obtained on CoCrMo testdisks: a) polished, b) after 30% HNO₃ treatment at 54° C. for 0.5 hr(representative of the conventional passive method) and c) after O₂plasma treatment with 1000 w, 300 mTorr, 250 sccm for 1 hr+30% HNO₃ at54° C. for 0.5 hr (an inventive process). The inventive treatmentresults in a substantially thicker surface oxide than the conventionallypassivated surface (about 5 nm vs 2 nm). In addition, the Cr/Co ratiomeasured for the surface treated using the inventive process (Cr isdominant and Co is near 0) is also enhanced relative to theconventionally passivated surface (Cr/Co=0.2).

FIG. 3 shows the same XPS sputter depth profile data as shown in FIG. 2,but the O and C profile data has been removed and the atomicconcentration for the metal atoms has been renormalized. a) polished, b)after 30% HNO₃ at 54° C. for 0.5 hr, (the conventional passive method)and c) after O₂ plasma treatment with 1000 w, 300 mTorr, 250 sccm for 1hr+30% HNO₃ at 54° C. for 0.5 hr (an inventive treatment). It shows thatCr/Co ratio in the inventive treatment surface oxide film (c) is thehighest.

FIG. 4 shows XPS sputter depth profile data obtained on CoCrMo testdisks: a) polished and b) after inventive treatment: O₂ plasma treatmentwith 1000 w, 300 mTorr and 250 sccm for 1 hr+6N HCl treatment for 1 hr.

FIG. 5 shows the XPS sputter depth profile data on the test disks: a)after 1 hr O₂ plasma treatment with 1000 w, 300 mTorr and 250 sccm+1 hr6N HCl treatment at room temperature, b) after treated with abovemethod, the surface was put into 6N HCl at 60° C. for 0.5 hrs, and c)after treated with above method, the treated surface was put into 6NNaOH at 60° C. for 1 hr. The inventive passive film is corrosionresistant in both acid and alkaline solutions.

FIG. 6 shows Hip simulator wear graph comparing the parts after 3million cycle wear tests. The one after inventive treatment (30% HNO₃,at 54° C. for 0.5 hrs+O₂ plasma treatment, 1000 w, 300 m Torr, 250 sccmfor 1 hr+30% HNO₃, at 54° C. for 0.5 hrs) is about 3-fold reduction inthe break-in wear than the one after the conventional passivationtreatment (30% HNO₃, at 54° C. for 0.5 hrs). The data shows that the ionrelease on the passivated surface of the invention is significantlyreduced.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention relates, in part, to chromium containing articlesand devices having thick and Cr highly enriched surface oxide layers.The invention also relates to processes to make improved wear surfaceson chromium containing materials. This surface preferably has astrengthened passive film, which can reduce ion release rates bydecreasing the corrosion rate. Wear is believed to be reduced byprotecting the wear surface from selective corrosion in surroundingcarbide areas. One type of process described herein includes plasmaoxidation and acid treatment. The acid treatment can be conducted afterplasma oxidation or both before and after plasma oxidation.

Certain processes involve the steps of subjecting a cobalt- andchromium-containing implant material to an energetic, oxidative gasplasma and treating the implant material with acid. This acid treatmentcan be achieved after plasma oxidation or before and after plasmaoxidation. The plasma treatment process can be achieved using anoxidative gas such as oxygen or a mixture of oxidative gases and inertgases at a power of between 100 and 1000 watts for a time of between 5minutes to 120 minutes. Compared with the oxide layer obtained from theconventional HNO₃ passivation process, the one obtained from theinventive process preferably is thicker and the Cr/Co ratio is higher.The enhancement of the oxide is believed to reduce the amount of metalion release due to corrosion and/or wear.

In certain methods of the invention, surfaces are treated with a gasplasma under conditions effective to oxidize at least a portion of thesurface. The oxidative component of the gas can comprise oxygen, water,hydrogen peroxide, or mixtures thereof. In some embodiments, theoxidative gas plasma can be formed, for example, from oxygen, a mixtureof oxygen with air, or with one of the foregoing along with one or morenon-reducible gases. Non-reducible gases include argon, nitrogen, neon,helium, xenon, and krypton. In some embodiments, argon is preferred. Insome preferred embodiments, the amount of oxygen in the gas is greaterthan 3%. In some preferred embodiments, the amount of oxygen in the gasis 3%-100%

Typically, a plasma is generated by creating an electrical discharge ina gaseous atmosphere under suitable pressure conditions. A typicalplasma treatment system has a chamber capable of being maintained at adesired pressure (typically atmospheric or below). The surface to betreated is placed with the chamber with the selected gas and anappropriate electrical discharge is applied to the chamber. A number ofgas plasma treatment systems suitable for use in the practice of thepresent invention are commercially available and such systems aregenerally known.

The energy source used in the gas plasma oxidation is one capable ofsupplying a sufficient amount of energy to ionize at least a portion ofoxidative gas to form a gas plasma. At least three power sources havebeen widely used in practice to supply the energy. These include DCelectrical energy, radio frequency (RF) energy, and microwave energy. Inmethods of the present invention, any suitable energy source, includingthe three mentioned herein can be utilized. It is generally recognized,however, that RF energy sources typically have the greatest sensitivityand are the freest from interference. Thus, in some embodiments, a RFenergy source is preferred.

The amount of energy utilized to form the plasma is typically from about100 to about 1000 watts. In some preferred embodiments, the amount ofenergy is from about 800 to about 1000 watts.

The gas plasma treatment of the surface is typically performed for about5 to about 120 minutes. In some embodiments, the time is from about 30to about 90 minutes. In other embodiments, however, the time is at least60 minutes.

The pressure at which the gas plasma oxidation is performed is typicallyfrom atmospheric pressure to sub-atmospheric pressure. In someembodiments, the pressure is from about 200 mTorr to about 400 mTorr. Incertain embodiments, the pressure is about 300 mTorr.

During the gas plasma oxidation, the flow rate of the gases through theoxidizing chamber is generally from about 100 standard cubic centimetersper minute (sccm) to abut 500 sccm. In one preferred embodiment, theflow rate is about 150-350 sccm (250 preferred in some embodiments).

In some embodiments, the acid includes nitric acid (HNO₃), hydrochloricacid (HCl), sulfuric acid (H₂SO₄) or citric acid. In some embodiments,the preferred acid includes nitric acid and HCl. The surface ispreferably contacted with a liquid acid at a concentration that suitablypassivates the surface. One preferred concentration range of the acid isabout 2M-8M In certain embodiments, the acid concentration is about3M-6M. The surface can be treated for a time sufficient to develop athick robust Cr rich surface layer. In some embodiments, the surface iscontacted with the acid for about 20 to about 120 minutes (30-90 min insome embodiments, or about 60 min in other embodiments). The treatmentcan occur at a temperature of 10° C. to 90° C. In certain embodiments,the preferred temperature is about 20° C. to about 60° C.

In some embodiments, more than one acid passivation step can beutilized. The acid passivation step can be conducted after plasmaoxidation or conducted before and after plasma oxidation. As usedherein, the term “before” includes intervening events that may occur(for example, “before” need not be “immediately before”). Certainembodiments have more than one acid treatment step after the plasmaoxidation. For example, an acid treatment step using an acid comprisinghydrochloric acid followed by another acid treatment step using nitricacid can be utilized.

“Passivation”, according to ASTM A380, is “the removal of exogenous ironor iron compounds from the surface of stainless steel by means of achemical dissolution, most typically by a treatment with an acidsolution that will remove the surface contamination, but will notsignificantly affect the stainless steel itself” In addition, it alsodescribes passivation as “the chemical treatment of stainless steel witha mild oxidant, such as a nitric acid solution, for the purpose ofenhancing the spontaneous formation of the protective passive film.

In the context of corrosion, passivation is the spontaneous formation ofa non-reactive surface film that inhibits further corrosion. This layeris usually an oxide or nitride that is a few atoms thick.

In the context of CoCrMo implant materials the XPS sputter depth profilepresented in FIG. 2 b illustrates the typical surface and near surfacecomposition of a conventionally passivated material, i.e treated in 30%nitric acid solution.

“Ultra passivation” is the inventive process described here in wherebythe surface, CoCrMo for example, is first treated in an oxygen plasmafollowed by an acid treatment. The surface (near surface) chemistrytypical for materials treated using this process is shown in FIG. 2 c.The inventive passivation process produces a significantly thickerprotective surface oxide layer relative to the conventional process(FIG. 2 b).

By “surface,” in reference to a device or other article, it is intendedto mean the outermost region of the device or article. In some cases,the surface of a device or article can have a metal oxide layer up to 20nm thick. Some devices or articles can have a plurality of surfaceshaving different composition.

Surfaces suitable for treatment with the methods of the instantinvention include those that are found on implantable devices (such asknee and hip replacement prostheses) and/or otherwise contain cobalt andchromium. Some metal implants are fabricated from surgical gradecobalt-chromium-molybdenum (CoCrMo) alloys because these alloys showgood corrosion and wear resistance.

At various points in the processes of the instant invention, thesubstrate surface can optionally be cleaned using typical usual cleaningprocedures, such as degreasing with detergent or an alkaline solution.Ultrasonic cleaning in detergent, followed by ultrasonic cleaning inwater and drying, may degrease the substrate surface. In someembodiments, the entire implant is cleaned. In other embodiments, only aportion of the implant will be cleaned. One skilled in the art willreadily appreciate that there may be a desire to perform an initialcleaning step before treating the surface with the processes disclosedherein. It may be desired to clean the surface between steps of theprocesses and/or at the end of the processes disclosed herein. Somecleaning steps might involve rinsing with soaking in water and followedby drying the surface.

The primary action of this treatment process is to impart ahigher/improved corrosion resistance to the CoCr materials. The improvedcorrosion resistance results from the formation of a strengthened oxidelayer with a high Cr/Co ratio on the device surface. While not wantingto be bound by theory, we believe that the enhanced corrosion resistancealso leads to lower wear and/or metal ion release. We further feel thatthis treatment process should also improve the overall corrosionresistance of CoCr materials used in other applications. This benefitwould include non-articulating surfaces as well as metal on poly ormetal on ceramic applications.

The invention is illustrated by the following examples which areintended to illustrative and not limiting.

EXAMPLES

Except where noted, all tests were conducted on the ASTM F-1537 CoCrMopolished disks. The finished disks were 2 mm thick and 25 mm indiameter. The surface of the disks was polished to a mirror finish (0.1μm) then rigorously cleaned in an Alconox solution in an ultrasonicbath. This step was followed with two (2) successive 3 minute RO(reverse osmosis) water rinse treatments.

Plasma treatments were performed in a PVA TePla Model 7200 PlasmaProcessing System by placing the sample CoCr disks in an aluminum traylocated in the center of the chamber. Power settings for the plasmatreatments were variable between 100 to 1000 watts. The examplespresented here were conducted at 1000 watts (w), but 500 w and 800 wwere also found to yield similar results. After placing sample disksinto the plasma system, the chamber was evacuated to approximately 30mTorr. After the chamber was evacuated, oxygen (O₂) was introduced intothe chamber to 300 mTorr with a flow rate of 250 sccm. The treatmenttime used in the examples was 1 hour. The operation of the plasmatreatment process was conducted under computer control, which includedestablishing and maintaining the process gas treatment “recipe”.

X-ray photoelectron spectroscopy (XPS) data was acquired using aPhysical Electronics (PHI) Quantara SXM instrument. The system uses amonochromatic AlKα x-ray source that emits photons at 1486.6 eV. Theanode was run at a power of 45 watts (15 KeV). The typical base vacuuminside the spectrometer was 5×10⁻⁹ torr. The analysis area for the XPSdepth profile acquisitions was set at 200 μm. Sputtering for the XPSdepth profiling was accomplished using energetic (2.0 keV) Ar ionsrastered over a 2×2 mm area. The XPS depth profile data presented hereis shown relative to depth quoted in Angstroms (Å). The sputter rate wasdetermined using a 1000 Å SiO₂/Si calibration standard. The depthprofile is displayed as atomic concentration (A.C.) versus sputter depthin Angstroms (Å). The sputter depth values in the graphs are calculatedby multiplying the calibrated sputter rate (discussed above) and thesputter time. The A.C. values represent a normalized elemental atomiccomposition for each of the species listed. The numbers are calculatedbased on the XPS peak area and corrected with standard calibrationfactors resident in the data analysis software provided by the vendor.

Example 1

Clean and polished 25 mm diameter CoCrMo (F 1537) disks were placed onthe center rack of the plasma chamber. The plasma chamber was evacuatedto a base pressure of 30 mTorr then backfilled to 300 mTorr with O₂(flow rate=250 sccm). The CoCr test disks were treated at an RF powersetting of 1000 watts for 1 hour. After allowing the samples in thechamber to cool to room temperature, the chamber was backfilled withdry-N₂ and the test disks were removed. The plasma modified CoCrMo diskswere then submitted for passivation using 30% HNO₃ for 30 minutes at atemperature of 54° C. At this stage the surface chemistry of themodified test disks was evaluated using X-ray Photoelectron Spectroscopy(XPS). More specifically, the XPS analyses were conducted as sputterdepth profiles, which entail interleaving argon (Ar) ion sputtering withXPS analyses. The Ar ion sputtering interacts with the topmost surfaceatoms causing them to be removed at controllable rate. In the presentanalysis the sputter removal rate was calculated to be approximately 80Å/min relative to the SiO₂ standard. The resulting depth profile dataobtained for the modified test disks is shown in FIG. 2. This figurecontains XPS sputter depth profiles obtained for CoCrMo test disks ofpolished (2 a), treated with the conventional passivation method (30%HNO₃ at 54° C. for 0.5 hr) (2 b) and treated with an inventivepassivation method (O₂ plasma treatment at 1000 w, 300 mTorr, 250 sccmfor 1 hr followed by 30% HNO₃ at 54° C. for 0.5 hr) (2 c). The profiledata is displayed with depth (plotted as Å removed) in the X-directionand atomic concentration (A.C.) in the y-direction. The data provides a3-dimensional picture that shows the elemental composition of theuppermost atomic layers (approximately 10-40 nm) of the sample material.The top plot FIG. 2 a shows the XPS sputter depth profile representativeof a polished CoCrMo test disk surface. The data shows that the topmostsurface of this sample contains equal concentrations of Co (x) and Cr(▴), or the Cr/Co ratio on the surface is about 1. The data furthershows the presence of a very thin (approximately 1 nm) surface oxidelayer, which is evident from the rapid decline in the oxygen (▪) signal.At a depth of about 2 nm, the A.C. composition measured by XPS reflectsthe bulk stochiometry of the alloy. The XPS depth profile data in FIG. 2b shows the surface after conventional treatment. The thickness of theoxide layer is about 2 nm and Cr/Co ratio is about 2. The XPS profilecollected for the inventive process treated an “ultra-passivated” sample(FIG. 2 c) indicates that the passivation process used for this sampleproduced a much thicker oxide layer. In this instance the relative depthof the oxide layer is approximately 5 nm. More importantly, the datafurther shows that the composition of the surface oxide layer ispredominantly a Cr-oxide, and while not wanting to be bound by theory,is most likely Cr₂O₃. Again, this interpretation is based on theelevated concentrations of Cr (▴) and O (▪) within the topmost 4-5 nmcoupled with the fact that the cobalt signal (x) was well below 10%(atomic) over this same depth range.

The same three XPS sputter depth profiles previously shown in FIG. 2 arereproduced in FIG. 3, but in this case the O and C traces have beenremoved and the metal A.C. values renormalized. This figure is includedto more accurately illustrate how the inventive “ultra-passivation”process alters the metal atom composition of the near surface region.This figure clearly illustrates the fact that the natural oxide layer onthe polished CoCr material (FIG. 3 a) is thin and has only a slightenhancement of Cr in it; conventional passivation increases the oxidethickness and Cr/Co ratio in the oxide layer (FIG. 3 b), and the“ultra-passivated” (FIG. 3 c) has the thickest oxide layer and thehighest Cr/Cr ratio in the outer 4 nm of the sample.

Example 2

Clean and polished 25 mm diameter CoCr (F 1537) disks were placed on thecenter rack (ground potential) of the plasma chamber. The plasma chamberwas evacuated to a base pressure of 30 mTorr then backfilled to 300mTorr with O₂ (flow rate=250 sccm). The CoCrMo test disks were treatedat an RF power setting of 1000 watts for 1 hour. After allowing thesamples in the chamber to cool to room temperature, the chamber wasbackfilled with dry-N₂ and the test disks were removed. The plasmamodified CoCrMo disks were then immersed in 6N HCl for 1 hr at roomtemperature. At this stage the surface chemistry of the modified testdisk was evaluated using X-ray Photoelectron Spectroscopy (XPS). Morespecifically, the XPS analyses were conducted as sputter depth profiles.In the present analysis the sputter removal rate was calculated to beapproximately 80 Å/min relative to the SiO₂ standard. The resultingdepth profile data obtained for the modified test disks is shown in FIG.4. FIG. 4 contains XPS sputter depth profiles obtained for both thepolished and the treated with the inventive process samples. In thisexample, the inventive process used is O₂ plasma treatment at 1000 w,300 mTorr, 250 smmc for 1 hr+6N HCl for 1 hr treatment. The top plot(FIG. 4 a) shows the XPS sputter depth profile representative of apolished CoCr test disk surface. This data shows that the topmostsurface of this sample contains approximately equal concentrations of Co(x) and Cr (▴). The data further shows the presence of a very thin(approximately 10 Å) surface oxide layer, which is evident from therapid decline in the oxygen (▪) signal. At a depth of about 20 Å, theA.C. composition measured by XPS reflects the bulk stoichiometry of thealloy. In contrast to the data obtained for the polished sample shown inFIG. 4 a, the XPS profile collected for an “ultra-passivated” sampletreated with the inventive process (FIG. 4 b) indicates that thepassivation process used for this sample produced a much thicker oxidelayer. In this instance the relative depth of the oxide layer isapproximately 5 nm. More importantly, the data further shows that thecomposition of the surface oxide layer is predominantly Cr-oxide, mostlikely Cr₂O₃. Again, this interpretation is based on the elevatedconcentrations of Cr (▴) and O (▪) within the topmost 4-5 nm coupledwith the fact that the cobalt signal (x) was well below 10% (atomic)over this same depth range

Example 3

A further proof of the enhanced corrosion resistance of the materialafter “ultra-passivated” inventive process treatment: FIG. 5 a shows theXPS sputter depth profile of a CoCr test disk that was passivated withthe inventive process (O₂ plasma treatment at 1000 w, 300 mTorr, 250smmc for 1 hr+6N HCl at room temperature for 1 hr). After above treatedsample was put into 6N HCl at 60° C. for 0.5 hr (FIG. 5 b) or 6N NaOHfor 1 hr (FIG. 5 c), the oxide film on the surface has no significantchange. Thus, the CoCrMo surface after the inventive treatment can standcorrosive environment.

Example 4

FIG. 6 shows the wear test results from the femur heads treated with theconventional process (30% HNO₃ at 60° C. for 0.5 hr) and with theinventive process (30% HNO₃ at 60° C. for 0.5 hr+O₂ plasma treatment at1000 w, 300 mTorr, 250 smmc for 1 hr+30% HNO₃ at 60° C. for 0.5 hr)respectively. The wear test curves show that the femur head after theinventive treatment reduced 3 folds of wear when compared with the oneafter the conventional treatment.

Example 5

Table 1 shows the ICP-MS (Inductively Coupled Plasma Mass Spectrometry)data (ICP data) that is obtained from the 20 ml 90% Bovine serumsolution which immersed 8 CoCr disks for 7 weeks. The disks are 1 inchin diameter and 0.125 inch in thickness. The solution was placed on ashaker in a 37° C. incubator. The result shows that the disks treatedwith the inventive passivation reduced the total ion release to 26% ofthat of the disks treated with conventional passivation.

TABLE 1 Ion Release of the CoCr Disks after Different Passivation in 90%Bovine Serum Sample Co (μg) Cr (μg) Cr (μg) Total Ions (μg) after 0.1160.014 0.018 0.148 Conventional Passivation (CP) after Inventive 0.0170.007 0.015 0.039 Passivation (IP) IP/CP (%) 15 50 83 26

Example 6

Table 2 shows ICP-MS (Inductively Coupled Plasma Mass Spectrometry) dataobtained from immersing the contacting area of a CoCr stem (size 4) and32 mm head modular system in a 0.9% saline solution which for 16 days.The results show that the implant treated with the inventive passivationreduced the total ion release to 5% of that of the implant treated withthe conventional passivation.

TABLE 2 the Comparison of the Ion Release of the CoCr implants Treatedwith Different Passivation in Saline Solution Released Ions Cr Co MoTotal ions Sample (ug/ml) (ug/ml) (ug/ml) (um) Conventional Passivation(CP) 0.010 0.200 ND 0.210 Inventive Passivation (IP) 0.004 0.008 ND0.012 IP/CP (%) 40 4 6

1-17. (canceled)
 18. An chromium containing article having a surfaceoxide layer up to 20 nm thick, said surface oxide layer comprisingchromium and cobalt oxides where the atomic ratio of Cr/Co more than 3.19. The article of claim 18, wherein said surface oxide is 2.5-8 nmthick and Cr/Co ratio is 3-7.
 20. The article of claim 18 that isimplantable into a mammal.
 21. The article of claim 20, wherein saidarticle is a joint replacement prosthesis or component thereof.
 22. Thearticle of claim 21, where said article is a hip or knee replacementprosthesis.
 23. The article of claim 22, wherein said surface is at awear interface.
 24. (canceled)
 25. (canceled)
 26. (canceled)